You can ask "What prompted you to ___?" to find out why someone did something. Specifically, you're asking what event made the person decide to start doing that action.

For example, here's a short conversation:

A: So what prompted you to look for a new job after all of these years working there?

B: Well, they just hired a new manager, who is totally incompetent. I couldn't stand it any longer.

"What prompted you to ___?" is also a little more formal than asking "Why did you ___?"